Conventionally, an image forming apparatus such as a copier, a printer, a facsimile, or a multifunction peripheral includes a plurality of rollers for conveying a sheet member on which an image is to be formed. The rollers are driven by a driving motor. As the driving motor, a servo motor such as a DC brushless motor may be adopted. It is noted that there is known a configuration where a DC brushless motor is used as a driving motor that drives conveyance rollers for conveying a sheet from a sheet feed cassette (see PTL 1).
In general, in an image forming apparatus in which the servo motor is adopted as a driving source of the conveying rollers, a detector such as a rotary encoder for detecting the rotation speed or the like of the driving motor is provided. In addition, a motor driver is electrically connected to the servo motor, and a control device such as a CPU is electrically connected to the motor driver. The control device outputs a control signal indicating a commanded speed to the servo motor, to the motor driver. The motor driver generates a driving current by the PWM (Pulse Width Modulation) method based on the commanded speed indicated by the control signal and the real rotation speed indicated by the detection result of the detector, and supplies the generated driving current to the servo motor.
Meanwhile, as the control signal output from the control device to the motor driver, a pulse signal is used that includes a plurality of pulses whose pulse width corresponds to each commanded speed to the servo motor. In this case, the motor driver detects the commanded speed and the cycle of the pulse signal based on the rising edge or the falling edge of the pulse signal (hereinafter, the edges are referred to as pulse edges).